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  • How Many Batteries & Why Tutorial

    This question comes up a lot so I thought I would take a couple of minutes to explain How and Why.

    The how is really a simple formula. The first step and most crucial is to determine how many watt hours you will need in a 24 hour period. You then multiply that number by 1.5 which is a fudge factor to account for charge/discharge efficiency, power factor, and inverter efficiency. If no inverter is involved then use 1.3. This will give you your adjusted watt hours per day. So for example let’s say you will use 3000 or 3 Kwh per day. 1.5 x 3000 wh = 4500 or 4.5 Kwh per day.

    The second step is to determine how many days of reserve capacity are required. The minimum is 2.5 days and up to 14 days. Then multiply this by 2 (for 50% depth of discharge as you never ever want to discharge more than 50%). So let’s select the minimum of 2.5 days. 2.5 days x 2 = 5 days.

    Third step is to take the number of days determined from step two, and multiply by the adjusted daily watt hour usage. So in this example 4500 wh x 5 = 22,500 wh or 22.5 Kwh.

    The last step is to dived the watt hour figure in step three by the battery system voltage. So now we must decide on a battery voltage of 12, 12, 36, 48, 60 and so on with multiples of 12 volts. For consumer grade applications the highest voltage (limited by equipment available to the public) is around 48 to 60 volts. Commercial applications can go as high as 500 volts and higher with special exemptions by limiting access to only qualified personnel. The voltage you choose is restricted mainly by the charge controller current capacity vs the solar panel wattage. Basically you want to use as high of voltage as you can afford to minimize power losses on the wring, and to keep the wiring as small as possible to minimize cost. For example the largest charge controller current available today to consumers is 80 amps. These controllers can be used on 12, 24, 36, and 48 volt systems. So a typical MPPT charge controller of 80 amps will have solar panel wattage limitations. For example at:

    * 12 Volts max panel wattage = 1000 watts
    * 24 Volts = 2000 watts
    * 36 Volts = 3000 watts
    * 48 volts = 4000 watts


    So for this example let’s assume we live in Kansas City which has a winter Sun Hour Insolation of 3.3 Sun Hours. So from another calculation (found elsewhere) we know we have to have minimum solar panel wattage of 1400 watts. So in this example the minimum battery voltage we can use is 24 volts. Now we have all the information we need to determine the battery Amp Hour Capacity needed. The formula is Watt Hour Capacity / Battery Voltage so using the numbers from previous steps AH = 22,500 / 24 volts = 938 Amp Hours.

    Fourth step is to select a battery. If possible we want to only have one single string of batteries wired in series to obtain the voltage needed. We only want to use true deep cycle batteries made for renewable energy. This limits manufactures, and you will not find them at Walmart. A very good manufacture with the best warranty is Rolls-Surrette. They have a very good selection tool you can use. Check the RE battery option, input Desired AH, input +/- Percentage (10 to 15%), and 20 Hour Rate. Then you will see your choices to the right. Select a battery with enough AH capacity to construct with 1 string if possible. In this case the Rolls S1380 is the right choice. It is a 2 volt @ 1050 AH battery so you would need 12 of them wired in series to make 24 volts. The battery carries a 7 year warranty, with 2 years free replacement and last 5 prorated. You can expect 5 years of life out of it with excellent care.

    Finally you might be asking why do I need so may batteries and reserve capacity. Well the answer is batteries only have so many cycles (discharge/charge) in them and the number of cycles depends on the depth of discharge. Generically the Cycles vs Depth of Discharges (DOD) look like this:

    * 50% = 200 cycles
    * 40% = 500 cycles
    * 30% = 1000 cycles
    * 20% = 2000 cycles
    * 10% = 4000 cycles

    Hope this helps.

    SK
    MSEE, PE

  • #2
    well done

    Along with he battery sizing, are parallel connections on the diagonal

    reasons and math are covered quite well here: http://www.smartgauge.co.uk/batt_con.html
    Last edited by Mike90250; 05-29-2012, 06:09 PM. Reason: added
    Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister

    Comment


    • #3
      First post

      I'm just starting to explore the off-grid solar world and was looking for a Energy calculation worksheet.
      Jeff Kammerzelt

      Comment


      • #4
        Go to the battery design link below
        The gaisma link is where you would get the insolation for the location.

        Before you start buy and use a kill a watt meter. Run it for a few days on anything you will have in the house.
        Pay particular attention to things like entertainment items like TV and cable boxes anything electronic that has a clock or light that stays on etc.
        these will be vampire loads that consume electric even though they are supposedly off.
        NABCEP certified Technical Sales Professional

        [URL="http://www.solarpaneltalk.com/showthread.php?5334-Solar-Off-Grid-Battery-Design"]http://www.solarpaneltalk.com/showth...Battery-Design[/URL]

        [URL]http://www.calculator.net/voltage-drop-calculator.html[/URL] (Voltage drop Calculator among others)

        [URL="http://www.gaisma.com"]www.gaisma.com[/URL]

        Comment


        • #5
          When working out the battery capacity required you can subtract the hours when sunlight is sufficiently powering the load via panels.
          In some cases where there are say ...4hrs...this would introduce a 15-20% sizing error if a simple kw/24 formula was used?

          Comment


          • #6
            No. calculations are made for a 24 hour period.
            Last edited by Mike90250; 10-07-2016, 03:10 AM. Reason: added period and space to No. calc
            MSEE, PE

            Comment


            • #7
              Originally posted by zed View Post
              When working out the battery capacity required you can subtract the hours when sunlight is sufficiently powering the load via panels.
              In some cases where there are say ...4hrs...this would introduce a 15-20% sizing error if a simple kw/24 formula was used?
              You can consider daytime loads separately, but if the battery is not sized to handle them, you should probably have a way to shut those loads down on cloudy days to prevent over-discharge. Making the calculations as Sunking has suggested will be much less likely to get you in trouble.
              CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

              Comment


              • Indanao
                Indanao commented
                Editing a comment
                We are building a 3080W 48V System in Mindanao. I am curious as to why 12V Deep Cycle car batteries cannot be used for a Off Grid house?
                Last edited by Indanao; 10-06-2016, 11:32 AM.

            • #8
              There is no 12V deep cycle car battery. There are Deep Cycle batteries, and there are Car Batteries.

              Car batteries have many thin plates, deep cycle batteries have fewer, but very thick plates.
              Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
              || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
              || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

              solar: http://tinyurl.com/LMR-Solar
              gen: http://tinyurl.com/LMR-Lister

              Comment


              • Indanao
                Indanao commented
                Editing a comment
                Not too sure about that... I have a Deep Cycle Car Battery in two of my cars. You can run em flat, and they will charge back up. Normal Batteries won't. So, since there ARE Deep Cycle Car Batteries ( with big plates ) why can't they be used for Solar Storage...was the question.

              • ButchDeal
                ButchDeal commented
                Editing a comment
                you can't run any lead acid battery "flat" without shortening its life. There are car battery systems with electronics to prevent the battery from being run to low if left on but that has nothing to do with the battery. There are batteries that are half way between as well but that is just that, and not as effective for deep cycle.

              • SunEagle
                SunEagle commented
                Editing a comment
                Some people actually use those Yellow and Blue top Optima batteries in their car. Technically they are considered deep cycle but they will still not live long if they are discharge too low.

                @Indnao. What is the make and model of your Car Deep Cycle battery?

            • #9
              When I'm in Canada, my LandRover diesel won't start when it gets down to zero degrees. I have run it flat 10 times, ( right dead ) boosted it off my other car, and it came right back. I guess all batteries deteriorate some each time used and recharged.

              Comment


              • ButchDeal
                ButchDeal commented
                Editing a comment
                running your battery flat cuts the life. The longer it is left flat the more life is cut. your indication that it will not start in cold weather it is showing its life. Don't run it flat and it will last a decade, run it flat and you might get a few years.

            • #10
              Your right. But, the reason it won't start in Cold Weather ( and it never has ), is because it isn't an AGM Battery. An AGM Battery will sustain max CCA ( cold cranking amps) until it dies. I'm guessing then that the answer is - because it won't last.

              Just curious as to your personal solar experiences - and if anyone who's tried it can say what to expect. I expect to have to buy 6 12V 250amp Storage Batteries for our 3080W system (11 280W Panels) from China. They quoted me $219 CNF Cebu per piece.

              Comment


              • ButchDeal
                ButchDeal commented
                Editing a comment
                I thought you were building a 48V system?? why would you buy "6" 12V batteries for a 48V system? did you mean 8 or 4?
                also you said you are getting 11 modules? bad idea. don't get a prime number of modules. What CC are you using? most likely you are going to want strings of 3 so you would want 12 modules.

            • #11
              Well, for 4 in series, and 2 in parallel off of two of them. Just for the storage.

              Comment


              • ButchDeal
                ButchDeal commented
                Editing a comment
                HUH!?? you can't put 4 12Volt batteries in series to make a 48V bank and parallel that with two 12V batteries. This can not be done.

            • #12
              Yes, on the Panels. Bought them cracked in shipping by UPS from a Solar Company on Ebay for $660. There were 11 on there way to Cebu. Sounds like I may need one more. Not quite sure what CC is...combiner?

              Comment


              • ButchDeal
                ButchDeal commented
                Editing a comment
                CC is charge controller. what is the charge controller.
                you will need a fused combiner as well though but that is separate.

            • #13
              Interesting. Well you could put 2 24V batteries in series with 2 12V Batteries ?

              Comment


              • #14
                No..i guess that would make it a 72V system. I still don't see why not? If you put 2 12V in Parallel you have 12V....many amps.?

                Comment


                • ButchDeal
                  ButchDeal commented
                  Editing a comment
                  you should probably start your own thread. You have a lot to learn and this sticky thread is not the place for it.
                  For now plan on 4 or 8 batteries at 12V each or 8 at 6V, 6 will not work.

                  You can plan on using 9 of the modules or get one more. they should all be tested to make sure they are working.
                  You will need an MPPT Charge controller like from outback Power or midnight.
                  you will need an electrician or some serious studying.

              • #15
                We planned a 48V Controler, mmp type.

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